I don't want to imagine how much time you spent on researching usable materials.

Bob

When doing imaginary "never seen" scenes---particularly those featuring people---I try to use individuals who were actually around at the time. Having grown up in the '60s I recall quite a bit of the shows and films that were out then. And, of course, I have some shows and films from that era on DVD which often helps when I need a screen grab. The image of Barbara Bain for instance is from a first season Mission: Impossible episode.
The aliens in the Pike era scenes are from The Outer Limits.

Sometimes I get hit with an inspiration, but often enough I envision something myself and then I have a reasonably good idea of where to find what I need.

Sometimes a suggestion or a reference spurs me. For instance I understand that Martin Laundau was once considered for the role of Spock. So now I'm thinking of Landau guest starring as a Vulcan during TOS' run. Someone upthread mentioned Ed Asner as a Klingon---I'd never thought of that---and it's an interesting suggestion so I'll look into it.

Other personages I'm thinking about:
James Franciscus
Greg Morris
Lee Remick
Lee Marvin
Honor Blackman (I did one of her previously, but like some images I've done before I'd like to revisit it and do it better)

The challenge when inserting people into a scene is not only getting people sized proportionately correct but also matching postures and shadows